Power combining techniques for radio frequency signals, including millimeter wavelength signals, have been accomplished in either a waveguide circuit or in a microstrip circuit. For example, prior art waveguide combining has been accomplished by feeding two or more signals in phase into a waveguide combiner. Although this type of power combining is efficient, the summing network is generally bulky and requires very high precision components. Microstrip power combining circuits have been accomplished by summing signals using a hybrid combiner circuit or a Wilkinson power summer circuit as known to those skilled in the art. This type of power combining circuit is more simple to implement in practice, but generally has higher losses.
FIG. 1 illustrates a typical waveguide combiner 20, widely available in the industry, and traditionally used to combine radio frequency signals from two sources of RF power. The combiner 20 can be formed from different materials as known to those skilled in the art, and generally has two input ports 22 that are bolted or fastened by other techniques to respective waveguide sources. The signals combine and are summed at the output port 24. This combiner 20 provides a reliable method of adding radio frequency energy, but requires careful phase matching of two radio frequency inputs and precisely control over the length of the two waveguide sides 26. The precision requirements for this waveguide and the requirement for a metal coating on the inside surface of the waveguide to achieve low losses results in relatively expensive devices. Also, this waveguide combiner is usually bulky, as illustrated, and occupies a significant amount of space.
FIGS. 2–4 show typical microstrip power combiners formed from microstrip transmission lines. These type of combiners are widely used in the industry for combining radio frequency power in microstrip circuits. There are primarily two types of microstrip combiners, using Wilkinson and hybrid circuits, as shown in the schematic circuit diagrams of FIGS. 2 and 3, respectively. The Wilkinson combiner 30 shown in FIG. 2 is a reflective combiner and includes two inputs 32, an output 34, and the Wilkinson circuit 36 that has a resistor for circuit balance as known to those skilled in the art. The hybrid combiner 40 shown in FIG. 3 is absorptive and includes two inputs 42, an output 44, and load resistor 46, forming a four port hybrid combiner. FIG. 4 illustrates a plan view showing the microstrip transmission lines 48 forming the circuit. In the hybrid combiner 40, the load resistor 46 absorbs any reflected energy because of mismatch. Typically, the three decibel (dB) Wilkinson combiner 30 results in 0.5 dB loss, while the hybrid combiner 40 results in 0.8 dB losses. These combiners provide a reliable method of RF energy summing and can be used in a very small space.
Other examples of various types of combiners and different RF coupling systems are disclosed in U.S. Pat. Nos. 4,761,654; 4,825,175; 4,870,375; 4,943,809; 5,136,304; 5,214,394; and 5,329,248.
As is also known to those skilled in the art, in a waveguide-to-coaxial line connector, a maximum energy field is in the center of the waveguide. An extension of a center conductor can be located at the point of a maximum energy field and act as an antenna to couple energy from a coaxial line into a waveguide. Coupling from a coaxial line to a waveguide could be achieved by using a loop, which couples two magnetic fields. In a prior art waveguide circuit using stripline or microstrip, the center conductor of a stripline can be extended into a waveguide forming a probe (or launcher). By increasing the width of a center conductor at the end of a probe, bandwidth can be improved. Also, the conductor and substrate of a microstrip circuit, but not a ground plane, can be extended directly into a guide.
In a prior art coaxial line circuit using a microstrip connection, the center conductor of a coaxial line can be pressed against or soldered to a conductor of a microstrip. The outer conductor of a coaxial line can be grounded to a microstrip ground plane. The microstrip substrate thickness could be as little as 0.010 inch for frequencies above 15 GHz, and usually requires decreasing the diameter of the coaxial line. In yet other types of systems, various directional couplers have waveguides that are located side-by-side or parallel to each other, or crossing each other. Stripline and microstrip couplers can have main transmission lines in close proximity to secondary lines. Although these examples can provide some power combining and coupling, they are not useful for combining two or more sources of radio frequency energy in a microstrip-to-waveguide transition with low losses or small “real estate” at an efficient rate at low power loss.